The present invention relates to an improved process for producing an integrated circuit arrangement including an infrared sensitive silicon substrate having an epitaxial layer and integrated electronic processing devices applied thereon, and an improved integrated circuit resulting therefrom. Such processes and devices are known in general.
At a given temperature, bodies emit thermal radiation corresponding to Plank's law of radiation with a frequency distribution which is characteristic for the particular temperature. The predominant portion of the radiation lies in the infrared wavelength range. There thus exists the possibility of obtaining a thermal image of the body by detecting its thermal radiation. For this purpose, the thermal radiation emanating from the body must be recorded by a suitable sensor. For practical application, the infrared sensors must be sensitive in one of the atmospheric windows. Practically only the wavelength windows, or ranges, from "3.mu. to 5.mu." and from "8.mu. to 14.mu." are applicable for the thermal image technique and various detectors which operate in these wavelengths ranges exist.
One class of thermal detectors operating in the above wavelength ranges includes, for example, PbSe, PbTe or CdHgTe compounds. However, due to almost unavoidable inhomogeneities in the composition of the material of these detectors, they exhibit great fluctuations in sensitivity even among themselves so that, for recording the thermal or temperature distribution of a body, almost exclusively only a single infrared sensitive sensor of this type can be employed in practice. Moreover, in order to compose an image, the body is scanned by means of a system of mirrors and lenses, the radiation is focused on the detector, and the photocurrent of the detector is a measure for the adsorbed light energy. For this purpose, the detector must be cooled so that its own thermal noise as well as the electronic noise in the detector system lies sufficently below the actual signal. To obtain fast image composition, it is desirable to arrange a plurality of detectors in a row to form a detector array. In this way, it is not necessary to scan the body in one dimension. A matrix arrangement, in addition to even faster image composition, would provide the additional advantage that no movable mirrors are required. However, in that case the detector homogeneity must be very high which, as already mentioned, cannot be realized with the above materials.
More promising than the above-discussed detectors seem to be detectors made of silicon material provided with suitable doping substances to enable them to detect infrared radiation. Such doping elements are, depending on the required wavelength range, gallium, indium, thallium, tellurium and the like. Such detectors have been operated with success both individually and in rows. Moreover, K. Numedal and coworkers (Huges Aircraft Company, Culver City, Calif. 90230) presented a matrix arrangement of infrared sensitive detectors on extrinsic silicon ("Proceedings of the International Conference on the Application of Charge-Coupled Devices"). These applications as well require sufficient detector homogeneity.
Inhomogeneities originate in silicon from the radially outward diffusion of doping atoms during the cooling phase when silicon crystals are drawn out of the melt. However, with the use of sufficiently high pressures during crystal drawing, this redistribution can be prevented. Moreover, a remainder of boron atoms, which is unavoidable in silicon, can be compensated with a corresponding concentration of phosphorus atoms. Such additional phosphorus doping can be accomplished relatively easily by way of irradiating the infrared sensitive silicon with thermal neutrons. After neutrons capture of silicon nuclei, the silison isotope, which has a known thermal cross section and is unstable with respect to beta decay, yields a phosphorus nucleus.
Compared to the above-mentioned infrared sensors, the detectors on a silicon basis have the decisive advantage that they can be integrated relatively easily with the necessary electronic devices for signal processing. A plurality of methods known from the semiconductor art can be used for this purpose.
However, infrared sensitive silicon substrates and the silicon materials employed in semiconductor electronics have such different properties that the integration of infrared (IR) detectors with the associated electronic signal processing devices is only conditionally possible on extrinsic silicon. This problem is circumvented in that a silicon epitaxial layer of suitable thickness and doping is applied to the photosensitive silicon starting material or substrate. In that case, the electronic signal processing then takes place in the MOS (or possibly also in the bipolar) technique, while employing charge coupled devices (CCD). The novel problem is now to establish electrical contact between the IR photoconductor and the electronic signal processing devices.
According to the prior art, the electrical connection between the IR photoconductor substrate and the integrated electronic signal processing devices is realized by means of a diffusion (annular) through the epitaxial layer into the photosensitive substrate. Under certain circumstances, it is necessary to effect an additional diffusion of low penetration depth between the contact diffusion for the IR detector and the initial diffusion for signal processing so as to assure this connection even for spatially far separated components (detectors, electronic circuitry).
Because of this contact diffusion, the geometric fixing of the detector range poses problems. Moreover, in practical operation, part of the photons impinging in the direction of the detector is already absorbed in the epitaxial layer before reaching the actual detector region and thus is lost as a contribution to the photocurrent.
A further, even more significant difficulty of this diffusion technique of detector connection to the electronic devices is the undesirable redistribution of the doping atoms. That is, the doping atoms used for the detection of light in the infrared wavelength range have relatively high diffusion coefficients so that a redistribution of the doping substances results during the high temperature steps inherent in the above-mentioned diffusion process, with this redistribution of the doping substances being considerable under certain circumstances. Particularly in the above discussed connection diffusion, with the relatively long diffusion time and the high temperature at which the diffusion takes place, this redistribution may have a particularly disadvantageous effect. In particular, doping atoms from the substrate diffuse into the oppositely doped epitaxial layer, while simultaneously the doping of the upper layers of the infrared sensitive extrinsic silicon substrate is changed by doping atoms from the epitaxial layer. This adversely affects the quality of the epitaxial layer and consequently also its electrical properties. Under certain circumstances, the redistribution of the doping atoms may even cause an electrical short circuit between the detector region and the electronic signal processing devices. Thus the contact diffusion constitutes a significant problem for the later operation of IR sensor elements and their associated signal processing devices on a single chip.
As can be seen from the above discussion, the required epitaxial layer thickness for a device of the type described above is determined by two contradictory requirements: on the one hand, the epitaxial layer should be sufficiently thin to assure absorption and thus detection of IR photons in the actual detector zone, i.e. the substrate, while on the other hand, no short-circuit must form between the epitaxial layer and the subtrate at the voltages occurring in CCD operation and in the resulting space charge zones underneath the individual electrodes. In the prior art, the thickness of the epitaxial layer is about 10.mu.. Such a device is disclosed, e.g., in an article by A. J. Steckl, "Low Temperature Silicon CCD Operation", Proceedings of the 1975 International Conference on the Application of Charge-Coupled Devices, pp. 383-388 and in an article by D. M. Erb, K. Nummendal, "Buried Channel Charge-Coupled Devices for Infrared Applications", Proceedings of the CCD Application Conference (Sept. 1973), pp. 157-167.